Electrolytic method for producing magnesium alloys



Dec. 2, 1947. A. YERKES ELECTROLYTIC METHOD FOR PRODUCiNG MAGNESIUMALLOYS Filed Jan. 17, 1942 ELECTROLYTIC cgu.

PURlFICATION CELL Fig. 3

INVENTQR BY Leland A. Yerkes ATTORNEY Patented Dec. 2, 1947 UNITEDSTATES PATENT OFFICE ELECTROLYTIC METHOD FOR PRODUCING MAGNESIUM ALLOYSClaims.

amended April 30, 1928; 370 0. G. 757) This invention described hereinmay be manufactured and used by or for the Government for governmentalpurposes, without the payment to me of any royalty thereon.

This invention relates to electrolytic deposition of magnesium from amolten bath and more particularly to increasing the solubility ofmagnesia and the like in a molten bath containing magnesium halides.

Numerous attempts have been made to electrolyze magnesia in moltenfluoride-oxide mixtures to obtain magnesium, e. g., U. S. 1,408,141;1,408,142; 1,310,449; 1,584,689; and 1,533,911. These processes have notbeen fully successful,

since magnesia has but low solubility in suchbaths (about 0.1% MgO;Thompson. M. de K.. and Kaye, A. L. Trans. Electrochem. Soc. vol. 67, 22pp., preprint 1935, The Decomposition Potentials and conductivities ofMgO, and Alkaline Earth Oxides in Fused Fluorides).

The principal object of my invention is the provision of an improvedprocess for producing magnesium.

A further object is to increase the magnesium oxide content of a moltenelectrolytic bath, whereby to facilitate electrolysis and recoveries ofmagnesium.

While the invention may be practiced in various specific Ways, and bythe use of many types of apparatus, I show in the drawing one practicalembodiment wherein Fig. 1 is a sectional view through a suitable type ofcell,

Fig. 2 is a diagrammatic view showing a cycle which may be used, and

Fig. 3 is an illustrative electrical installation.

Referring now generally to the invention, I have discovered that boroncompounds, preferably when utilized within certain limits in themagnesium halides containing bath, will materially increase thesolubility of magnesia therein, and that this increased solubility ofmagnesia makes more practical the electrolysis of thismagnesia-containing bath to form magnesium metal. This increasedsolubility of the magnesia makes MgO more nearly the product reduced inpractice, possibly since magnesia, (according to Thompson and Kaye citedpreviously) has a lower decomposition voltage than has the magnesiumhalide MgClz. The amount-of boron compound,

calculated as B203, to give increased solubility of the magnesia mayvary from traces to an upper limit of about 5% of the bath. At highconcentrations of boron, the electrolytic production of magnesium borideobscures or prevents the reduction of magnesium metal. I have found thata satisfactory practical percentage is about 0.5%, although, as stated,up to about 5% maybe used to advantage. Even 0.1% to 0.3% may be-used toadvantage.

The bath may contain any alkaline earth or alkali metal halide inaddition to the magnesium halides. For example, a bath of NaCl, MgClz,B203 and MgO may be utilized. The use of salt and similar halides in thebath is usually for the purpose of rendering the bath more fluid orlowering its melting point.

Magnesium oxyhalides, such as magnesium oxy chloride, magnesiumchlorate, magnesium'hypochlorite, magnesite, brucite, etc., may besubstituted for magnesia. This bath may be used for electrolyticpurposes, either to produce magnesium directly by usual practice or toplate into other metals, e. g. lead, either to form a useful alloy orone which can be subsequently refined to give magnesium metal.

The halide of magnesium preferably employed is magnesium chloride, asthe basis of the bath. Into this is placed a boron compound, suchasB203, and such MgO as can be dissolved therein. The electrolyticproduction of magnesium is then carried out by methods old in the art.

The process employed is preferably a continuous one, although some ofthe features of my invention may be employed with equal advantage if theelectrolysis is carried out on a batch basis. Using the continuousprocess, purified MgO is fed to the reduction cell along with such MgClzand B203 (or other boron compound) as may be required, whileelectrolysis continues. Chlorine or oxygen or both are liberated at thesurface and metallic magnesium is formed at the cathode. When a leadcathode is used, the magnesium mixes with the molten lead. Thelead-magnesium mixture is circulated from the reduction cell to aseparation furnace. The magnesium may-be removed from the lead byelectrolysis of the molten lead-magnesium alloy to yield metallicmagnesium, as more fully described by Jessup "in British Patent No.268,737. -The lead product resulting from this separation, which stillcontains some magnesium, is returned to the reduction cell. Continuedoperation may result in building up impurities in the electrolyte orlead cathode in which event operations may be discontinued to renew orpurify the constituents of the reduction cell.

Referring now to the drawings, I show in Fig. 1 the cell I with arefractory lining 2. An anode 3 projects into the electrolyte layer 4which floats on top otthe molten cathode 5; e. g. lead. A lne 6comprises an outlet for withdrawing the leadmagnesium mixture forseparation, and the lead with reduced magnesium content is returnedthrough the return line 1 comprising aninlet A suitable purification orseparation cell oriurnace 8 receives the lead-magnesium mixture, reducesthe magnesium content, and returns thelead to the reduction cell, a pump9 of suitabledesign being employed to circulate the'load. The cell 9 andgeneral arrangement of Fig. 2 are described by Jessup, in the patentreferred to hereinabove.

Electrolysis of the magnesium oxide to-magnesium requires direct currentof controlledvoltage; A motor generator set U may be employed,operatingfromcommercial linevoltage, and delivering power at suitablevoltage to-the cell-through the-anode 3; andcathode, The-cell may beheatedby resistance, I2-i through transformer [3, although otherheating;means; may-be employed-.-

Atl timeslt may-be necessary tocoolthe cell, in which case any suitable:cooling: means. (notmakingfrequentadditions of-MgO.-: The voltageaveraged 4.5 with the current at150 amperes during-most: of the. period,but was. later increased to: 6.'5 volts .at. 135 amperes. Thecurrentdensity" was-.20 amperes per square inch-at -50 amperes and .35.:amperesper square inch at 135'amperes. The. gas givenoff atrthe. anode .wasanalyzed. The following table is.illustrative. of, the results:

Amperes Volts Temp. MgQrIn Cell 4.5 1.9L 814' 10%": 59 l h y s a e he 0%4., 82 57 856 Excess MgQ. 4-.5 11.88- 840 Excess-not all dissolved thatwould dissolve; 6.5 63.00 816. Excess.

I have already-pointed out thatcertain features; of cellconstruction-and operation heretofore suggested by others may beemployed in the practioe of my invention, Instead of a lead cathode;other-molten: metals may be used such as tin, zinc-and bismuth, fromwhich magnesium may besuitably-separated. Uther-typcs of cells-may b'eused: With the particular design of cell shown in -the-drawing,- it --isdesirable that a carbonaceous-anode be used; such as. azgraphite ansrgases evolved are particularlycorrosive tometal 1 anodes it directlyexposed to them. fiinee I agraphiteanode isburned by the gases e olved;toCO and CO2, it-isdesirable also that it be readily-replaceable.-

irrthe-practice of my invention, many commundsofberon may be used tosecure thee- 1* vantageous results described. Examples other than B203,are borax, borax glass, ulexite, boracite, colemanite. kernite. boricacid and the like, Boron oxide, B203, however, is the preferred boroncompound to use, so far as I have observed from results heretoforeobtained. This com-- pound increases markedly the solubility of MgO in aMgClz bath, and the rate of solution is such that MgO can becontinuously added. Other magnesium compounds are more readilyintroduced into the; solution as well; such as :magnesium oxyhalides,which by-heat arereduced to oxides and chlorides, and subsequentlyelectro- Iyzed in the same manner as is MgO.

It shouldbe understood that my invention is not limited by the specificdetails of construction and operation of a cell discussed above.

Any suitable electrolyte may be used, such as MgCl2: towhichotherelectrolyte salts have been added. By my invention MgO may be made, atleast for the most; part, the ultimate material reduced. The ratio ofoxygen to chlorine in the table shows that much-more oxide is reducedthan chloride, and "I-believe' that it is merely a question ofdetermining optimum conditions to substantially reduce only MgOand-practically no lVIgClz;

I: wish topoint out also, that-instead of, ern-- ploying a molten metal;such aslea-d forthecathode, molten magnesium itself may be usedasthe;

cathode providing asuitable cell construction is used'so thatMgOmay-beaddedand the evolvedgas, 02 and C12, canbe-JceDt outer-contactwith the metallic magnesium.

What I claim as new and desire to protectby Letters Patent of theUnitedStates'is:

1; A process which consists of subjecting to electrolysis a fusedmixture of'magnesium-oxide, magnesium chloride and from 0.1 to 5 .0%-calculated as B 03 of at least oneboron com-- pound, as a solubilitypromoter for M550, selectedfrom the group consisting ofgboro-n trioxide,boric acid, borax; borax glass, ulexite; beracite, colemaniteandkernite, said-electrolysis lee-- ing carried out While employing asamolten me tallic cathode at leastone-metal=capable ofalloyingwith-magnesium, saidmetal being selected from the group consisting-oflead, tin, zinc and bismuth, to produce an alloy of magnesium with saidmolten cathode metal.

2. A process which-consists of subjecting to electrolysis a fusedmixture of magnesium oxide,-

magnesium chloride and from 0.1 to 5.0%;o f -bo-.

ron trioxide, as. a solubility promoter-for MgO, said electrolysisbeingcarried out while employing molten lead as a cathode to producean alloyof magnesium with lead;

3. A process which consists of subjecting to electrolysis a fusedmixture of magnesium oxide, magnesium chloride and about 0.5% borontrioxide, as a solubility promoter for MgO, said electrolysis beingcarried out While. employing as a molten metallic cathode at least onemetal capae ble of alloying with magnesium, said metal being selectedfrom the group consisting of lead, tin, zinc, andbismuth, to produce analloy of magnesiurn with said m'olten cathode metal.

ep o ess which QnSist fb ec n to lec ly is a fu edix r of ma nesium oxde, magnesiumchloride andabout 0.5% calculated a S' B303 of atleast oneboron; compound, asa solubilitypromoter for MgO, selected fromthe, groupconsistin of boronv trioxide, boric acid, borax,- borax glassrulexite,boracite, colemanite and-'ksrnit said e Q trQ -Ydshe -carried out LELANDA. YERKES.

REFERENCES CITED The following references are of record in the file ofthis patent:

Number Number UNITED STATES PATENTS Name Date Kugelgen et a1 Feb, 25,1908 Ashcraft Aug. 16, 1921 Cottringer et a1 Dec. 29, 1925 Andrieux Mar.10, 1936 Schambra et a1 Feb. 12, 1945 Slansky Aug. 14, 1945 FOREIGNPATENTS Country Date France Apr. 8, 1935 OTHER REFERENCES Journal of theChemical Society (London), 1891, page 1321.

